UNIT 2

# Pharmaceutical Calculations:

Weight and measures

There are 2 systems of weight and measures,
Imperial system
Metric system

Imperial system:-
Avoirdupois system– Pound is the standard unit for weighting all measures of all mass are derived from it.(1 pound = 5760 grains)
Apothecaries system (Troy stem) – Grain is the standard unit in this system

Metric system:-

Measurement of weight – Kilogram is the standard unit of measurement(1 kg= 100 gm)
Measurement of capacity – liter is the standard unit of measurement (1 L = 100 ml)

## Calculations involving % solutions:-

Calculate the amt. of 95% alcohol required to prepare 400 ml of 45% alcohol.
Vol. req. = 400 ml, % alcohol required= 45%, % alcohol used= 95%
Vol. of strong alcohol to be used = vol. required * % required / % used
= 400*45/95
=184.47 ml

### Alligation Method:-

When calculations involve mixing of 2 similar preparations of diff. strength, to produce a preparation of intermediate strength, the alligation method is used.
Calculate the volume of 95 % alcohol required to prepare 600 ml of 70 % alcohol
Vol. required = 600 ml, % alcohol required = 70, % alcohol used = 95
70 parts of 95% alcohol & 25 parts water will produce the required % alcohol
Qty of 95% alcohol req. = 600*70/ 95 = 422.10 ml
Qty of water req. = 600*25/95 = 157.90 ml

Proof spirit Calculation:-
The strength of alcohol is calculated in proof degree. As per Indian standards 100% proof spirit = 57% v/v ethyl alcohol
The value of more than 57 % = overproof spirit
The value less than 57 % = under a proof spirit
Calculate the proof strength of 95% v/v alcohol.
Proof strength= % strength of alcohol* 1.753-100
=95*1.753-100
= 66.53° O.P

Isotonic solution:-
The solutions having same osmotic pressure or equal solute concentration is called an isotonic solution.
Freezing point:-
% w/v of adjusting substance needed = 0.52-PSM*a / b
Where, PSM= percentage strength of the measurement
A= freezing point of the unadjusted solution
B =freezing point of 1% w/v sol. of adjusted solution

Molecular weight:-
% w/v of adjusting substance needed = 0.03M/N
Where M= gram molecular weight
N= no. of ions into which substance is ionized

###### Pharmaceutical Calculations Notes

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